Coating of iron and steel



May 18, 1965 A PRUST 3,184,326

COATING OF IRON AND STEEL Filed June 10, 1960 I I'l ll l I'll W ALVIN F. PRUST IN VEN TOR.

HIS ATTORN Y United States Patent Oiiice 3,184,325 Patented May 18, 1965 3,184,326 COATING OF IRON AND STEEL Alvin F. Prust, Warren, Ohio, assignor to Republic Steel Corporation, Cleveland, Ohio, a corporation of New Jersey Filed June 10, 1960, Ser. No. 35,326 12 Claims. (Cl. '117--51) This invention relates to improvements in the formation of aluminum coatings on iron and steel. More particularly, it relates to an improved process and improved pretreating composition for forming a uniform and welladhered coating of aluminum on the iron or steel.

An aluminum coating on iron and steel provides excellent protection against corrosion. However, although numerous methods have been proposed for applying such coatings, they leave something to be desired for commercial use, either because of erratic or unsatisfactory results in the process and product, or because of the expense involved in the materials used or the method of application. Furthermore, toxic fumes from some of the materials used in certain processes have helped to make such processes even less attractive.

Some of the proposed methods utilize a molten aluminum bath in which the steel is immersed after it has been treated in molten salt fluxes. Another suggested method calls for keeping the surface of the steel wet with water until it is to be immersed in molten aluminum.

Another proposed method involves a pretreatment with an aqueous solution or suspension of sodium fluosilicate, potassium chloride, sodium chloride, and sodium fluoride, or cryolite. A more recent suggestion recommends pretreating with an aqueous solution of zinc chloride together with various other chlorides.

In spite of the variety of salts and the variety of combinations of such salts recommended as pretreating fluxes and the various methods of pretreatment recommended, the proposed methods are not particularly suited to commerical use either because of expense, complicated procedures, or ineflicient results.

It has now been found that an aluminum coating can be applied to iron and steel very efliciently and inexpensively by applying to a freshly cleaned iron or steel surface, preferably at a temperature of about 170-l90 R, an aqueous solution containing approximately 1.3-6.0 ounces of an alkali metal fluoride, such as sodium fluoride, and approximately 1.3-6.0 ounces of a stannous salt, such as stannous chloride, stannous fluoride, stannous sulfate, etc. per gallon of solution, together with suificient acid to keep the pH between 3.5 and 4.0 (0.6 to 2.8 ounces of 22 Baum hydrochloric acid per gallon of solution), removing excess solution, drying the aqueous film on the metal, and then introducing the metal into molten aluminum. The process is adaptable to batch, semi-continuous, or continuous operation.

While it is not intended that the invention be restricted to such a theory, it is believed that the stannous salt and the alkali metal fluoride react to form a complex fluoride, such as Na SnF This complex fluoride is in a form which is resistant to hydrolysis in the presence of water and forms a chemical-barrier layer on the surface of the metal. This thin, chemical-barrier layer dispenses with the need for a heavy deposition of the various fluxes previously used which generally resulted in porosity Although other acids, such as this purpose, it is generally preferred to use hydrochloric acid to maintain the desired pH.

Before the iron or steel surface is treated, it must be cleaned entirely of foreign materials, such as oxide coatings, greases, etc. Various pickling procedures can be used for this purpose. A preferred method of cleaning is to immerse the iron or steel for approximately 15 to seconds in about 8-10% sulfuric acid at about F. Any equivalent method of obtaining a clean surface is satisfactory.

Various types of steel can be used in the process, ineluding various carbon steels, silicon steels, low alloy high-strength (such as those containing 1.0% copper, 1.0% nickel, 0.15-0.20 molybdenum), etc. The iron or steel can be in various shapes, and either unfabricated or fabricated articles can be used in the process. In continuous modifications of this process, the steel advantageously is in strips, ribbon, wire, or other form adapted to be stored in roll form prior to being advanced through the various processing steps. Also, fabricated articles can be run through the sequence of treating steps on a conveyor device.

Although the various proportions of the salts will operate in accordance with the range indicated above, it generally is advantageous to have approximately equal parts by weight of the stannous salt and the alkali metal fluoride. A preferred composition has 3.0 ounces of sodium fluoride and 3.0 ounces of stannous chloride per gallon of solution, with suflicient hydrochloric acid therein to give a pH of 3.5-4.0. Approximately, 1.4 ounces of 22 Baum hydrochloric acid per gallon of solution gives the desired pH range. While higher concentrations of these'salts also are operative, these concentrations are the most economical.

Various methods of applying the aqueous liquid film to the metal surface can be used. Where convenient the metal object can be immersed in the liquid and excess liquid allowed to run off the article after its removal from the bath. Sheets, ribbons, wires, or filaments of the metal can be run continuously below the surface of the aqueous bath and excess liquid allowed to run oif after the metal has passed through the bath, or can be squeezed off by passing the metal between two rollers. The aqueous liquid also can be applied to the surface of the metal by flowing the liquid over the metal, or by spraying the liquid thereon.

While it is generally desirable to have both the metal and the solution approximately at a temperature in the range of about 170-190 B, it is possible also to have one or the other outside this range, providing the resultant temperature of the flux and metal during the contact time is approximately the temperature indicated. Treating bath temperatures as high as 400 F. have been found satisfactory. An immersion .or contact time of 6-10 seconds generally is satisfactory to give the desired flux film.

After the metal has been in contact with the aqueous solution, it is, necessary that the resulting aqueous film be dried on the surface of the metal by evaporation of the water as soon as possible and prior to having the metal pass over any roll other than the squeegee roll used to remove excess liquid. Various methods of drying the aqueous film can be used provided the film is not rubbed off the metal surface. Radiation from heated surfaces or heat lamps and currents of warm air are found particularly advantageous for this purpose.

Within ten minutes after drying, and preferably as soon as possible after drying, the resultant metal surface should be immersed in the molten aluminum. Abrasion of the metal surface should be avoided as muchas possible although very minor abrasions will not disturb the dried film sufficiently to prevent aluminum coating.

The flux of this invention isnot sensitive to exposure to ambient atmospheric conditions; Strip has been coated commercially satisfactory as to porosity and formability,

after being exposed to air up to 20 minutes between fluxing and dipping in the. aluminum bath. It has been ascertained further that strip orarticles coated with thisflux can be preheated before immersion inthe' molten aluminum metal'with no deleterious effects upon the resultant coating. In this way, less heat. input to the. molten alu minum pot is necessary. V

A thickness of film sufiicient to give complete coverage without any pinholes or bare spots. is desired. Such coverage is obtained by observing the conditions described herein. Since complete reaction between the molten aluminum and the dried filmis desirable, a salt film of excessive thickness is not desirable. of -15 seconds in the molten aluminum bath is. generally satisfactory, although a shorter period of 3 or 4 seconds is sometimes desirable. V V

The temperature of the metal strip entering the aluminum bath is generally not over 150 F. However,

according to the temperature developed .during' the sub sequent drying step. i

The temperature of the aluminum bath'advantageously is sufliciently above the melting temperature of the aluminum oraluminum alloy therein so as toIcompens atefor heat loss to the metal passing through thebath as well as to normal losses to the surroundings; However, for economy of heat and of time in cooling the coated product, it is generally not desirable tomaintain a molten aluminum An immersion period this can vary according to the flux bath temperature and V temperature greatly in excess of that required to keep the aluminum in a molten condition.

Generally a temperature in the range of about 1275 F. to 1330 F., preferably aboutlZSOl-lZQO." F. is economical and satisfactory.

When the. aluminum bath is maintained at a 'temperagui de rolls-1 2" and 12', into a molten aluminum bath 13 sheet of one mode of carrying out the process.

Then the sheet 1 coming from the aforesaid rinsing stage or from a storage roll, is passed over guide roll 2 into a treating bath 3 in container 4 of such dimensions as tov give the metal approximately a 10 second residence time in the bath.. The sheet is passed under a second guide roll '5. The bath contains 3.0 ounces of sodium fluoride, 3.0 ounces of stannous chloride, and 2.5 ounces of 22 Baum hydrochloric acid per gallon of solution. The pH is checked periodically and additional acid added when necessary to maintain a pH in the range 'of 3.5-4.0. The bath temperature is maintained in the range ofapproximately 170 F. to 190 F.

After leaving the aqueous bath, the sheet is passed throughfrolls 6 to remove excess aqueous film, then passed between'two banks of heating lamps 7 to effect drying of the film. Guide roll 8 is spaced far enough from the banksof lamps that the water is completely evaporated from the film before the sheet comes into contact with it. Stack 9 is provided in enclosure 10 for escape of the moisture laden air.; If desired, forced ventilation can be provided. I l I Then the sheetis passed over guide roll 11 and under 5 maintained atxa temperature of approximately '1280" ture substantially below 1280""F., the metal strip cools H the bath at the ,area where it enters the bath so thata semi-solid mushy deposit buildsupinthis area.. At an 7 aluminum bath temperature of133( F5, the resultant productrhas an oxidized surfac e,indicated by a gold "cast. While neither of these effectsis'completely objectionable, it is desirable to stay at least five degrees within the extremes of this temperature range, preferably. about 1280-1290 F. i r I Where it is desired to coat the iron or steel-With an aluminum alloy, 'suchias for example, aluminum silicon,

the corresponding aluminum alloy molten bath will be used and the temperature modified accordingly. 7

After themetal is coated it iscooledby exposure to the feet before' the sheet comes in contact with a guide roll. If desired, forced air coolingcanbe effected. When the product is to'be rolled; or coiled, thecoated' atmosphere. "It is generally desirable to allow at least 256 '7 product can'have a temperature as high as about .400? F. 7

although temperatures ofl120-165f F. are generally-advantageously etfectedi This temperature is determined by the temperature of the metal atentry to the cooling chamher, the temperature of thecoo'ling medium, and the riodof cooling; o r r r .7 Atypical pretreating and; coatingoperation with a roll lation tov the schematic flow-sheetfshown in the accent.-

' F. and of such dimensions-that the sheet has a residence time of approximately'5 seconds.

. After emerging from the molten aluminum bath, the

"sheet passes upward througha distance of about 25 feet and is cooled by exp os ure to the atmosphere so that the temperature is reduced to approximately 12095165" F.

Then, after passingjover guide'roll '14,fthe cooled, coated product is rolledand stored on the: driven storage roll 15.

The resultant product is found to have excellent adhesion of'the aluminum coating to the metal base and upon fabricationinto various shapes exhibits excellent adhesion of the coating to the base without any cooling or cracking of the coating. Similar continuous processing -ofiron and steel wire and ribbon gives-similar coated 'products which can be benton an extremely small radius without causing" cracking or peeling of the coating. Strips coated in'this manner can becold rolled to a bright luster and can be drawn or shaped by any conventional means without flaking of the aluminum'co'ating;

The-foregoing description is merely illustrative and various modifications are obvious.

;While certain features of this invention have been described in detail with respect to various embodiments thereof, it will, of course,- be'apparent that other modifications maybe made withinthe spiritand scope of this inventionand it is not intended to limit the invention to the of 0.060 auge cold-rolled steel-is described below-inrepanying drawing illustratingfia continuous coating process; .1 Y

according to the practice :of'this invention.- {The roll is.

so as to move'thesheet forward; "The shee't'then is passed overguide-rolls and then into 'ajpic'kling bath ofsuchJ dimensions that'the metakhasaresidence time therein ij of approximately '60 seconds;

I 'Ihejpickling bath is about;8-1;0% sulfuric'a cidymaintained at 'l80200,3 F. "Afterthe pickling-bath, the sheet" suppo tedin such a manner that fit is freefto rotate on its axi t t e she Pa n h u h'd ve r n ro s.

' exact details'shjown above except insofar as they are defined in the following claims; The invention claimed is I 1. Anaqueous 'pretreatir g solutionfor improving the adhesion of an aluminum coating' to .a ferrous vbase consistinglessentially of about l.3T6.0 ouncesof alkali metal fluoridejxand about .1;3.f6,0 ouncesof a stannous salt per gallonof faqu'eousf solution maintained at a. H/in the range-ofabout3.574.117

iw ydrochloric acid j 'j2i1'A soluti0nof claim .liin which'said stannous salt is stannous chloride and said fluorideis sodiumfiuoride.

' 3; A solutionof, claim 2 inwhichsaid is'maintained 4-. A solution of claim 2 in which said proportions of said sodium fluoride and said stannous chloride are approximately equal.

5. A solution of claim 4 in which said pH is maintained with hydrochloric acid.

6. A process for applying an aluminum coating to a ferrous metal comprising the steps of applying a film of an aqueous solution to a ferrous metal surface, said solu tion consisting essentially of about 1.3-6.0 ounces of an alkali metal fluoride and about 1.3-6.0 ounces of a stannous salt per gallon of aqueous solution, said solution having a pH in the range of about 3.5-4.0, immediately thereafter evaporating the water from said solution, and then immersing said dried metal surface in a molten aluminum bath for a period suflicient to apply a coating of desired thickness.

7. A process of claim 6 in which said stannous salt is stannous chloride and said fluoride is sodium fluoride.

8. A process of claim 6 in which said pH is maintained with hydrochloric acid.

9. A process of claim 7 in which said sodium fluoride and said stannous chloride are present in said solution in approximately equal proportions by weight.

10. A process of claim 9 in which said pH is maintained with hydrochloric acid.

11. A process of claim 10 in which said aqueous film is applied at a temperature in the range of approximately 170190 F.

12. A process of claim 6 in which said aqueous film is applied at a temperature in the range of approximately 1'70190 F.

References Cited by the Examiner UNITED STATES PATENTS 527,478 10/94 Broadwell 1175 1 2,671,737 3/54 Jominy et al 117-51 2,706,161 4/55 Westby 11752 3,027,269 3/62 Teshima et a1. 1171 14 X RICHARD D. NEVIUS, Primary Examiner.

JOSEPH B. SPENCER, Examiner. 

6. A PROCESS FOR APPLYING AN ALUMINUM COATING TO A FERROUS METAL COMPRISING THE STEPS OF APPLYING A FILM OF AN AQUEOUS SOLUTION TO A FERROUS METAL SURFACE, SAID SOLUTION CONSISTING ESSENTIALLY OF ABOUT 1.3-6.0 OUNCES OF AN ALKALI METAL FLUORIDE AND ABOUT 1.3-6.0 OUNCES OF A STANNOUS SALT PER GALLON OF AQUEOUS SOLUTION, SAID SOLUTION HAVING A PH IN THE RANGE OF ABOUT 3.5-4.0, IMMEDIATELY THEREAFTER EVAPORATING THE WATER FROM SAID SOLUTION, AND THEN IMMERZING SAID DRIED METAL SURFACE IN A MOLTEN ALUMINUM BATH FOR A PERIOD SUFFICIENT TO APPLY A COATING OF DESIRED THICKNESS. 